Sensitive infrared dectectors are now used on satellites to detect the thermal radiation from objects under observation. These instruments are designed for the detection of very low levels of radiation intensities and usually are cryogenically cooled. Therefore, they saturate at rather low radiation levels and cannot be used to observe the earth's limb, or in directions near the sun. Also, because of the fairly large collecting optics, which also have to be cryogenically cooled, these detectors are vulnerable to high intensity radiation from existing high energy lasers. In this case some protection can be obtained by cutting off the electronics whenever the rate of increase in intensity exceeds a certain value. But, it is difficult to determine when it is safe to again turn on the electronics, and the detector may also be damaged beyond recovery even when the electronics are turned off, if the radiation intensity is large enough. These detectors operate in certain wavelength bands like 3 to 5 .mu.m, 8 to 14 .mu.m or 16 to 22 .mu.m, and filters are used to prevent radiation at other frequencies from reaching the detector surfaces.
The instrument disclosed here uses a gas absorption cell in which the attenuation to wavelengths of interest is controlled by the output of the detector itself. In this manner the dynamic range of the detector can be extended many orders of magnitude, thus, allowing the detector to be used to determine where the high intensity radiation is coming from and also to measure the intensity of the radiation reaching the collecting optics. The continuously variable attenuator may be utilized either in front of or behind the collecting optics. In either case the cell will have to be cryogenically cooled and the normal filters may be used as end windows for the cell. However, if the cell is used behind the collecting optics then the cell can be made quite small. The intensity reaching the instrument is calculated by an on-line computer from the measurements of the pressure in the absorption cell and the intensity reaching the detector by use of Beard's law; I = I.sub.o e.sup.-.alpha.p. Where I is the intensity reaching the detector, p is the pressure in the absorption cell and .alpha. is absorption coefficient of the gas used in the attenuator. Normally .alpha. is a constant, but in some cases, like the use of SF.sub.6 near 10.6 .mu.m, .alpha. is a function of both I and p, [.alpha. = .alpha. (I,P)], and would also have to be calculated by the computer.